This application claims the benefit of Japanese Application No. 2000-126050, filed Apr. 26, 2000, which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a line pressure control device that controls a line pressure applied to a continuously variable transmission (hereinafter referred to as xe2x80x9cCVTxe2x80x9d) provided in a vehicle.
2. Description of Related Art
In recent years, a CVT provided in a wide variety of vehicles is constructed in such a manner as to supply a primary pulley and a secondary pulley, which are connected to one another through a belt, with hydraulic fluids provided by an oil pump to thereby adjust the effective diameters of both pulleys and transmit a driving force from an engine to driving wheels at a speed ratio corresponding to the driving condition of a vehicle. To prevent a belt slip caused by the transmission of power, the pulleys always apply a clamping force to the belt by receiving the line pressure provided by the oil pump. To prevent the belt slip, the line pressure should be set to a high level so as to apply a strong clamping force to the belt. The increase in the line pressure, however, leads to a loss of energy generated by an engine that is driving the oil pump. It is therefore desirable to adjust the clamping force by controlling the line pressure according to a torque that is transmitted between both pulleys (i.e. a torque inputted to the CVT).
For example, Japanese Patent Publication No. 5-48386 discloses a line pressure control device that calculates an engine torque from a throttle angle and an engine speed, and sets a target line pressure according to the calculated engine torque (regarded as the torque inputted to the CVT) and a speed ratio of the CVT. A margin is allowed for the target line pressure due to a potential calculation error, a mechanical error of the CVT and the like. If, however, the engine torque rapidly increases due to sudden operation of an accelerator pedal, for example, the hydraulic pressure control to increase the line pressure of the CVT cannot follow the increase in the engine torque. As a result, the line pressure becomes lower with respect to the engine torque, and a clamping force corresponding to the engine torque cannot be applied to the belt. This leads to the temporary shortage of the clamping force and results in the belt slip.
If the margin for the target line pressure is increased in expectation of the above situation, the oil pump loses an excessive amount of energy in a normal state. To solve this problem, a line pressure control device disclosed in Japanese Patent Provisional Publication No. 4-50440, for example, transitionally suppresses the engine torque until the change in the line pressure follows the increase in the engine torque. This aims at preventing the belt slip without increasing the margin for the target line pressure
The engine torque is suppressed by retarding an ignition timing and the like, but it""s impossible to suppress the engine torque in every operational condition. For example, if a cooling water temperature is low, the retardation of the ignition timing causes unstable combustion. If an engine speed lies in a high speed range, the retardation of the ignition timing abnormally increases an exhaust temperature and causes damage to catalysts. In other words, the engine torque cannot be suppressed under some operational conditions. Therefore, the line pressure control device disclosed in the above publication is unable to surely prevent the belt slip when it is impossible to suppress the engine torque.
It is therefore an object of the present invention to provide a line pressure control device and method for a continuously variable transmission, which is able to surely prevent a rotary element of the continuously variable transmission from slipping.
To attain the above object, the present invention provides a line pressure control device for a continuously variable transmission having a first rotary element and a second rotary element in contact with the first rotary element, comprising: a hydraulic pressure control unit for adjusting a line pressure operating on the first rotary element of the continuously variable transmission; a line pressure control unit for setting a target line pressure based on an output torque of an engine and controlling said hydraulic pressure control unit such that an actual line pressure matches the target line pressure; and an engine output torque suppressing unit for determining, whether the output torque can be sufficiently suppressed, such that the first and second rotary elements can be prevented from slipping, based on an operating condition of an engine, and temporarily suppressing the output torque when the output torque can be sufficiently suppressed, wherein said line pressure control unit coercively increases the target line pressure when said engine output torque suppressing unit determines that the output torque cannot be sufficiently suppressed.
The present invention also provides a line pressure control method for a continuously variable transmission having a first rotary element and a second rotary element in contact with the first element, comprising: setting a target line pressure based on an output torque of an engine; controlling an actual line pressure such that an actual line pressure matches the target line pressure; determining whether the output torque can be sufficiently suppressed, such that the first and second rotary elements can be prevented from slipping, based on an operating condition of an engine; suppressing the output torque when the output torque can be sufficiently suppressed; and coercively increasing the target line pressure when the output torque cannot be sufficiently suppressed.